With the emergence and proliferation of wireless technology, the Internet today has evolved to a stage where numerous data communications end-points are made up of mobile terminals, each roaming through different domains and attaching itself to different points of attachment to the packet-switched data communications network (such as, the Internet) at different points in time. Such roaming provisioning is fairly matured in a circuit-switched communications network, such as the phone system. In a packet-switched communications network, however, supporting such roaming capabilities is difficult. This is because terminals in a packet-switched communications network are reached using unique addresses, and such addresses usually contain portions (usually the prefix) that must be valid in a spatial topology. In addition, it is desirable for mobile terminals to continue to be reached at the same address after a plurality of change of point of attachment to the packet-switched data communications network. This allows seamless continuation of sessions (such as file transfer) across different points of attachment to the packet-switched data communications network.
To support such roaming capabilities, the industry has developed solutions for mobility support as addressed by Mobile IPv4 [the following Non-patent document 1] in Internet Protocol version 4 (IPv4) [the following Non-patent document 2] and Mobile IPv6 [the following Non-patent document 3] in Internet Protocol version 6 (IPv6) [the following Non-patent document 4]. In Mobile IP, each data communications terminal (called a mobile node) has a permanent home domain. When the mobile node is attached to its home network, it is assigned a permanent global address known as a home-address. When the mobile node is away, i.e. attached to some other foreign networks, it is usually assigned a temporary global address known as a care-of-address. Such a temporary address is usually assigned by the access router the mobile node is associated to, and it is aggregated at the address topology of the access router so that the care-of-address is topologically correct in the routing infrastructure of the global network. The idea of mobility support is such that the mobile node can be reached at the home-address even when it is attached to other foreign networks, so that other nodes in the packet-switched data communications network need only identify the mobile node by the mobile node's home-address. This is done in Non-patent documents 1 and 3 with an introduction of an entity at the home network known as a home agent. Mobile nodes register their care-of-addresses with the home agents using messages known as Binding Updates. The home agent is responsible to intercept messages that are addressed to the mobile node's home-address, and forward the packet to the mobile node's care-of-address using IP-in-IP Tunneling [the following Non-patent documents 5 and 6]. IP-in-IP tunneling involves encapsulating an original IP packet in another IP packet. The original packet is sometimes referred to as the inner packet, and the new packet that encapsulates the inner packet is referred to as the outer packet. Such a binding between home-addresses and care-of-addresses, made known at the home agent of the mobile node, allows the mobile node to be reached no matter where the mobile node is.
It is possible to extend the concept of host mobility support as stipulated in Non-patent documents 1 and 3 to network mobility support (NEMO) as defined in the following Patent document 1 and the following Non-patent document 7. This is the case where the mobile node is itself a mobile router, and a plurality of nodes move together with the mobile router, forming a mobile network. Packets sent to the mobile network are intercepted by the home agent of the mobile router and tunneled to the mobile router, which decapsulates the tunnel packet, and forwards the inner packet to the destination. Similarly, packet sent from a node in the mobile network is tunneled to the home agent by the mobile router to be forwarded to the correct destination. The mobile router itself may act as an access router, allowing other mobile nodes (mobile hosts or mobile routers) to associate with the mobile router and gain access to the global communications network through the mobile router.
However, a system using the aforementioned mechanism has one fundamental requirement, that is, a mobile node must maintain link layer connectivity with the access router the mobile node associates with. Because the access router relays all packets sent to or from the mobile node, once the mobile node loses link layer connectivity with its access router, it can no longer send or receive packets. Thus, in such a kind of mobile network, the access architecture is hierarchical in nature, where a mobile node obtains its care-of-address from its access router, and must maintain connectivity with its access router.
There exists a mechanism utilizing mesh network protocols that allows a mobile node to forward packets to other mobile nodes within the same vicinity. Such a kind of mobile network is of a mesh nature, and typically employs Mobile Ad-Hoc Networks (MANET protocols). For example, the following Patent document 2,discloses a method for a mobile node to distribute link information to neighboring nodes, so that each mobile node can discover routes to any other mobile nodes in a mesh-like fashion. The following Patent document 3 discloses another example of a routing method where the associativity characteristic of neighboring links of a mobile node is used to disseminate routing information. In all this examples, two mobile nodes can send data packets to each other even if there is no link layer connectivity between them. The packet is sent via one or more intermediate nodes that do have link layer connectivity with each other. However, it remains unclear how a mobile node in a wireless mesh network can communicate with a node in the wired global communications network. To this end, the following patent document 4 has proposed a method where mobile nodes used Mobile IP to communicate with other nodes in the global network, and packets is relayed between the mobile node and an entity known as the Foreign Agent using MANET protocols.                Non-patent document 1: Perkins, C. E. et. al., “IP Mobility Support”, IETF RCF 3344, August 2002.        Non-patent document 2: DARPA, “Internet Protocol”, IETF RFC 791, September 1981.        Non-patent document 3: Johnson, D. B., Perkins, C. E., and Arkko, J., “Mobility Support in IPv6”, IETF Internet Draft: draft-ietf-mobileip-ipv6-24.txt, Work In Progress, June 2003.        Non-patent document 4: Deering, S., and Hinden, R., “Internet Protocol Version 6 (IPv6) Specification”, IETF RFC 2460, December 1998.        Non-patent document 5: Simpson, W., “IP in IP Tunneling”, IETF RFC 1853, October 1995.        Non-patent document 6: Conta, A., and Deering, S., “Generic Packet Tunneling in IPv6”, IETF RFC 2473, December 1998.        Non-patent document 7: Devarapalli, V., et. al., “NEMO Basic Support Protocol”, IETF Internet Draft: draft-ietf-nemo-basic-01.txt, September 2003.        Patent document 1: Leung, K. K., “Mobile IP mobile router”, U.S. Pat. No. 6,636,498, October 2003.        Patent document 2: Elliot, B. B., “Distribution of potential neighbor information through an ad hoc network”, U.S. Pat. No. 6,456,599, September 2002.        Patent document 3: Toh, C. K. “Routing method for Ad-Hoc mobile networks”, U.S. Pat. No. 5,987,011, November 1999.        Patent document 4: Airiksson, F., et. al., “Mobile IP for mobile Ad Hoc networks”, US Patent Application 20010024443, September 2001.        
The hierarchical architecture in Mobile IPv6 and NEMO is more efficient in the sense that there is no need for each mobile node to maintain a routing table and no need for flooding of route updates as required by MANET protocols. This is especially crucial since mobile nodes are usually resource limited and battery-powered, thus any saving in power consumptions and memory usage is significant.
However, these kinds of protocols (such as Mobile IPv6 or NEMO) require one or more access routers acting as the central node to forward packets. Any mobile node that associates to these access routers must maintain link layer connectivity with the access routers. If the mobile node wanders too far away, it will lose all connectivity. It cannot even communicate with another mobile node that is within its wireless range, because it can't forward packet to the access router that should then forward the packet to the destination node (that may be right beside the sender).
The mesh network protocols such as MANET specifically address this problem, allowing mobile nodes to form a dynamic mesh network, whereby mobile nodes may forward packets sent by other nodes that are not destined for themselves. Routes are either advertised proactively or discovered on-demand, giving mesh network nodes the ability to adapt dynamically to the changing structure of the mesh. This feature of mesh network is highly desirable, since the mesh network thus can be self-healing. However, mobile nodes using mesh network protocols will incur some deficiency. To send a packet, a mobile node must spend some time to discover routes to the destination. Mobile nodes also consume power even when they are not sending packets, since they must participate in relaying packets sent from other mobile nodes. In addition, some amount of route information must be maintained in the memory.
It is possible for an existing equipment to make use of both hierarchical and mesh architectures as a simple combination of two mechanisms. When a hierarchical mechanism fails, the above-mentioned equipment will switch to a mesh mechanism, and vice versa. However, such a simple combination of the two architectures will result in disruption of on-going transport session, mainly because the addresses used in each of two architectures are usually different.